• The on-yomi of 虚(KYO, KO) is similar to that of 空(KŪ). Both on-yomi mean “empty.”

• The two characters even hook up in these compounds:

空虚 (kūko: emptiness, vacancy)虚空 (kokū: empty space, sky)

Jim Breen’s online dictionary gives “untruth” as another definition of 虚. Indeed, as Kenneth Henshall notes in A Guide to Remembering Japanese Characters,虚 (empty) combines with 口 (mouth) to form the character 嘘 (uso: lies). We’ve seen this kind of falseness in many 空 words.